1. Field of the Invention
The present invention generally relates to the art of semiconductor optics, and more particularly to a short period, strained-layer superlattice structure including at least two coupled quantum wells, which produces optical bistability when incorporated into a semiconductor laser as an active region thereof.
2. Description of the Related Art
The semiconductor laser, or laser diode, is a device which operates by using stimulated emission to produce a very intense monochromatic coherent light beam. It generally includes a P-N junction in a direct bandgap material which can be used to inject a minority-carrier density that is sufficient to invert the population on one side of the junction. Two opposite sides of the structure are flat and parallel and may be partially coated for increased reflectivity. When the injection level reaches inversion, a spontaneous recombination may emit a photon nearly perpendicular to the reflecting surfaces. This photon in turn can trigger another minority carrier to recombine, yielding a second photon propagating in the same direction due to coherence between stimulated and stimulating photons. As these photons traverse the active region, an avalanching photon emission takes place. Usable radiation is extracted through the reflecting surfaces.
The P-N junction, which constitutes the active region of the device, is sandwiched between two or more layers of material which serve to confine photons produced by stimulated emission and injected charge carriers in the active region. Generally, the confinement layers have a smaller optical index of refraction than the active region, thereby forming an optical waveguide which causes total internal reflection of photons in the active region and propagation of photons through the waveguide perpendicular to the reflecting surfaces in longitudinal and/or transverse modes in accordance with the particular design criteria.
Injection of charge carriers for stimulated emission is accomplished by means of electrode layers formed on the externally facing surfaces of the confinement layers (or intervening substrate/auxiliary layers if provided). A direct current or pulse voltage is applied to the electrode layers in a direction which causes the P-N junction to be forward biased. The confinement layers lave a larger bandgap energy than the active region, thereby enabling charge carrier injection into the active region and preventing the injected carriers from leaving the active region.
Optically bistable semiconductor lasers offer extremely high speed operation, are potentially switchable in the pico-second time scale, and may be employed as optical switches and optical logic gates in optical and optoelectronic integrated circuits. Optical computers are a primary application of this technology, but eventually it could replace many of the present electronic integrated circuits.
Optical bistability is essentially a hysteresis phenomenon in the optical power output vs. injection current curve. The laser has two possible output powers for the same input power (current). These two output powers can be either both above laser threshold or one above and one below it. In the former case, increasing the injection current beyond a first value causes population inversion and stimulated emission which turns the laser on. The laser will remain turned on until the injection current is subsequently reduced to a second value which is significantly below the first value. The difference between the first and second values is the hysteresis or "bistable region" of the laser. The device may be operated as a bistable switch by applying a steady-state injection current which is intermediate between the first and second values, and modulating the steady-state current with relatively small positive and negative switching pulses to turn the laser on and off respectively, or between 2 on states--a low and a high one. Numerous other devices, such as logic gates, latches, limiters, etc. can be constructed using a bistable semiconductor laser as the main element.
Optical bistability in semiconductor lasers has been accomplished in the past by various methods. A primary expedient has been to fabricate one of the electrode layers in a pattern which is separated into conductive and non-conductive areas. The non-conductive areas constitute saturable absorbers which produce optical bistability by means of inhomogeneous excitation. An example of a device of this type in which the contact area is in the form of a segmented stripe is disclosed in a paper by H. Kawaguchi entitled "Bistability and Differential Gain in Semiconductor Lasers", Japanese Journal of Applied Physics, Volume 21 (1982) Supplement 21-1, pp. 371-376. This paper also describes bistability induced by optical injection. Another electrode structure which has been demonstrated to produce optical bistability is a "twin-stripe" configuration reported in a paper entitled "Room-Temperature Optically Triggered Bistability in Twin-Stripe Lasers", by I.H. White et al, Electron Lett. (6B) 19. pp. 558-560 (July 1983).
Another structure which has produced optical bistability is an "external cavity", such as presented in a paper by P. Zorabedian et al entitled "Bistability in Grating-Tuned External-Cavity Lasers", IEEE Journal of Quantum Electronics, Vol. QE-23, No. 11, Nov. 1987, pp. 1855-1860.
The main disadvantage of inhomogeneously pumped bistable lasers (segmented stripe and twin stripe) is that they rely on unconventional contacts which require special processing and reduce the flexibility of the design and the ability to integrate the structure with other devices. Bistable lasers employing external cavities are bulky, and impractical for integration with other devices.
Semiconductor diode lasers employing single quantum well strained layer structures are known in the art as disclosed in "Strained-Layer InGaAs-GaAs-AlGaAs Photopumped and Current Injection Lasers", by R. Kolbas et al, IEEE J. Quant. Elec., 24. pp. 1605-1613 (Aug. 1988). However, the wavelengths attainable with such devices are limited to very small values, due to the thinness of the quantum well.
Semiconductor diode lasers including multiple quantum wells per se are also known in the art. One example is disclosed in U.S. Pat. No. 4,788,688, issued Nov. 29, 1988 to Thomas Hasenberg et al, entitled "HETEROSTRUCTURE LASER". However, the quantum wells in this example are so thick that they behave the same as bulk material.
Another example of a laser diode structure including multiple quantum wells is found in a paper entitled "Bistability in inhomogeneously pumped quantum well laser diodes", by A.I. Kucharska et al, IEE Proceedings, Vol. 135, Pt.J.No.1, Feb. 1988, pp. 31-33. However, optical bistability is achieved by means of inhomogeneous pumping, and the operation as the same as with a structure in which the multiple quantum well active region is replaced with a bulk GaAs active region.